Multiple component composite products used in high stress environments can often delaminate due to the high stresses. Some of the causes for such delamination are different coefficients of expansion in multiple layers of material, including different types of materials, and different stress levels in adjacent layers of materials.
One example of where these differences in coefficients of expansion of adjacent materials and differences in stresses in adjacent layers of material can occur is in helicopter rotor systems. The blades, which are often formed of composite material, and supporting root fitting structures, which are often formed of metal, could produce high stresses in the area of the juncture between the blades and the root fittings. Such high stresses could cause delamination of the composite material adjacent the rotor at the connection between the root fittings and blades. These delaminations can occur because of the different coefficients of expansion of the metal and composite, differences in stresses at the juncture of the composite and metal, or both. Many factors are involved in reducing the overall stresses in this area and in reducing the differences in stresses among the components. These factors include the shapes of the parts in the area of the juncture and in the formation of the composite doublers that are part of the blade skin that attach to the root fittings. It would be advantageous to be able to modify such junctures to reduce the overall stresses in the composite and thus extend the useful life of such structures.
A further problem with such structures is that delaminations may occur beneath the surface of the composite material where it attaches to the root fittings and may not be detected in routine inspections. The ability to detect the eminent delamination could reduce the risk of deterioration of the rotor system.
Aside from the rotor system example, there are many applications of composite materials in combination with other composite materials, or other materials such as metals, where the prevention and/or detection of delamination would be desirable.